258
Health Physics
the test site were in agreement with the meteorological
input data used by the model, there is no guarantee that
the model will predict correct depositions because the
meteorology downwindcan also be in error.
Significant differences in predicted fallout were
frequently observed when precipitation processing was
enabled as compared to when precipitation processing
was disabled. As shown in Table 2, the differences in
deposition were occasionally as great as a factor of three,
although generally much less. Often, the amount of
fallout was reduced at a given atoll when precipitation
processing was enabled, apparently due to cloud depletion at upwind locations. At other times, predicted
deposition was higher, suggesting local rainout or washout. The limitations in the HYSPLIT wet deposition
model (in common with other meteorological models),
and the lack of precipitation data on an event-specific
basis, may have contributed, in some cases, to the
quantitative differences seen between the measured fallout deposition and model simulated estimates at some
locations at some times.
Based on comparisons for tests where significant
monitoring data were available, predicted depositions
using the HYSPLIT three-dimensional particle model
coupled with NCEP/NCAR reanalysis meteorological
data agree with measured '°’Cs densities to within a
factor of ten. This was almost always true when the
initial wind speed and direction of the meteorological
input data agreed reasonably well with that of the actual
wind data at the test site.
Table 2. Comparison of HYPSLIT simulationsof '*’Cs deposition
density (Bq m7’) with and without wet deposition for selected tests
and atolls.
Wet deposition
disabled
Atoll/Test
Fir (11 May 1958)*
Kili
Ebon
Mejit
Flathead (11 June 1956)*
Namorik
Wotho
Nectar (13 May 1954)*
Namorik
Kili
Jabat
Lib
Lae
Dog (7 April 1951)*
Kwajalein
Ujelang
Lae
* All dates GMT.
Wet deposition
enabled
Atoll
domain
1 degree
domain
Atoll
domain
1 degree
domain
30
30
78
22
30
100
15
40
110
22
22
100
340
5,200
300
3,300
140
4,100
300
3,000
40
74
130
850
440
160
40
85
520
740
85
48
180
560
560
160
40
110
520
780
5.2
24
0
3.3
110
2.6
9.3
60
3.0
6.7
190
5.6
August 2010, Volume 99, Number 2
Inadequate meteorological data were generally the
limiting factor in the HYSPLIT model’s ability to predict
accurate arrival times of fallout for Marshall Islands
tests, although high-quality data on actual arrival times
were also often lacking due to few measurements having
been made and someinconsistencies between the available measurements (Beck et al. 2010). These conditions
made comparing model-based estimates with measurements
a difficult exercise. A comparison of model-predicted fallout arrival times with reported best estimates (Beck etal.
2010) is provided in Table 3.
As discussed, the HYSPLIT model uses a simple
rainout and washout model that may not adequately
simulate such complex processes, particularly for the
relatively large amounts of debris in nuclear test clouds.
Forthis reason and because of the normal high frequency
of precipitation events in the Marshall Islands, most of
which would not have been recorded in archival meteorological data sets, particularly in the southern atolls
(Beck et al. 2010) where rainfall is the highest, the
HYSPLIT simulations may not have predicted some
actual deposition events. Moreover, it is possible that
some of the differences in the HYSPLIT-predicted fallout arrival times as compared to the generally earlier
arrival times observed could be a result of fallout
deposited as a result of precipitation scavenging from
unrecordedprecipitation events.
Test-specific results. The results of simulations of
fallout from five nuclear tests in the Marshall Islands are
discussed here. Simulation results were compared against
existing measurement data when possible, although anecdotal reports of fallout from test participants were
considered as well. Further details on the characteristics
and dates of the tests are given in an appendix of a
companion paper (Becket al. 2010).
Greenhouse Dog was a pure fission device which
was detonated on Enewetak Atoll on 7 (GMT) April
1951. No radiological survey data are available for the
Dog test. However, the HYSPLIT simulations suggest
that small amounts of fallout could have occurred at
several atolls in the Marshall Islands including
Ujelang (Fig. 1), Wotho, Kwajalein, and Utrik. Modelpredicted wind speed and direction agree fairly well
with the observed values reported in DNA (1979)
(Table 4). Dog is an example where HYSPLIT simu-
lations wereparticularly valuable because there are no
historical monitoring data.
Greenhouse Item was a fusion device detonated on
Enewetak Atoll on 24 (GMT) May 1951. Similar to the
situation for Dog, no radiological survey data exist for
the Item test. The HYSPLIT simulation suggests there
was significant fallout at Ujelang Atoll (Fig. 1). Because